The mechanism of direct electrolytic reduction of SiO2 in molten CaCl2 was studied. Morphological and crystallographic investigations were conducted on Si prepared by potentiostatic electrolysis of SiO2 contacting electrode at 1.10 V (vs. Ca2+/Ca) for 1 h at 1123 K. X-ray diffraction confirmed that amorphous SiO2 was reduced to crystalline Si. From scanning electron microscopy (SEM) and energy-dispersive X-ray results, it was proved that Si columns were formed perpendicular to the reaction interface between Si and SiO2 and that vacant spaces were formed between the columns. It was found from field emission SEM observation that the Si column had basically a hexagonal prismatic and stacking structure. Transmission electron microscopy and electron diffraction results revealed that the Si column was a single crystal having {111} twin planes perpendicular to the axis of the column. It is explained that amorphous Si is first formed by electrochemical reduction and then thermally transformed to crystalline Si. The rate-determining step of the reduction was found to be O2- diffusion in the vacant space between the columns. (c) 2005 The Electrochemical Society. All rights reserved.